The electronic and optical properties of semiconductors are known to be dependent on the size of the semiconductor particles. For example, there is a minimum size which the particles must exceed before light absorption occurs at the bulk bandgap (i.e., before the polymolecular cluster becomes a semiconductor). The onset of bulk semiconducting properties of CdS has been estimated to occur for particles whose diameters exceed 60 .ANG.. For PbS, the band gap shifts to higher energy as the semiconductor cluster size decreases, and eventually converges to the transition energy of the first excited state of the PbS molecule; bulk semiconducting properties appear for particles whose diameters exceed 150 .ANG..
The preparation of small particle semiconductors has been pursued in an attempt to exploit the altered electronic and optical properties of these materials, relative to bulk semiconductors. However, the preparation of extremely small particulate semiconductors is often difficult and seldom applicable to a wide range of semiconductor compositions. Some semiconductors have been prepared and studied in the gas phase, low-temperature matrices, reversed micelles, surfactant vesicles, bilayer lipid membranes, clays and as colloid suspensions in solvents containing various surfactants to maintain the dispersions. However, the small-particle semiconductors prepared by these methods may be intrinsically unstable towards aggregation or difficult to incorporate into an electronic or optical device. For a useful device, the small-particle semiconductor should be incorporated in a solid, preferably transparent, medium which can be modified by standard fabrication techniques and which can provide an inert or protective environment for the reactive semiconductor material.
In one approach, Cd, S and Se have been added to the standard ingredients of normal glass to prepare CdS or CdS.sub.x Se.sub.1-x glass cutoff filters by standard melt procedures. Glasses of this type are commercially available as long-wavelength-pass optical filters, with several values for x. Nonlinear optical effects have been reported in these glasses, but the high temperatures and strongly oxidizing conditions used to prepare these glasses severely limit the applicability of this technique to other semiconductor compositions.
Mahler, Inorganic Chem., Vol. 27, Number 3, 1988, pp. 435-436, discloses additional preparative methods, including metathesis in microemulsion, gas-solid reactions on high surface area silica, synthesis within the channels of perfluorocarbon sulfonic acid membranes, and generation of semiconductor particles within polymer films. In particular, ethylene-15% methacrylic acid copolymer (E-MAA) was shown to provide good mechanical and optical properties and confer high kinetic stability on nanometer-sized semiconductor particles.
Rajh et al., Chemical Physics Letters, Vol. 143, No. 3, 1988, pp. 305-307, disclose a method for incorporating a quantized particles of colloidal semiconductors in transparent silicate glasses by mixing aqueous colloidal dispersions of the semiconductor with tetramethoxysilane (TMOS), accelerating the polymerization of the silicon alkoxide by the addition of NH.sub.4 OH, and drying the resulting gel over a period of months. They also disclose a method for producing colloidal glasses by first incorporating metal ions, and then, after drying to about one-half the original volume, adding the appropriate anions for precipitating the particles via gaseous H.sub.2 S or H.sub.2 Se.
Roy et al., in "Better Ceramics Through Chemistry", Materials Res. Soc. Symp., Vol. 32, Ed. J. C. Brinker, D. E. Clark, D. R. Ulrich, Elsevier, 1984, disclose the inclusion of CdS and AgX (X=Cl, Br, I) in sol-gel monoliths by mixing a tetraethoxysilane/ethanol solution with an aqueous solution of the heavy metal ion.
Kuczynski et al., J. Phys. Chem., Vol. 89, 1985, pp. 2720-2722, disclose the preparation of CdS in porous Vycor.RTM. glass by soaking cleaned porous glass in a CdCl.sub.2 solution, drying the glass under vacuum and then immersing the impregnated sample in a sodium sulfide solution.
The nonlinear optical properties of semiconductors such as degenerate four-wave mixing, optical bistability and phase conjugation have been reported (Rustagi et al., Optics Letters, Vol. 9, No. 8 (1984), pp. 344-346, and reference cited therein). Rustagi et al., describe an experimental arrangement for measuring degenerate four-wave mixing of visible radiation in a borosilicate glass doped with the mixed semiconductor, CdS.sub.x Se.sub.1-x.
The materials provided by the prior art, in which small-particle semiconductor particles are imbedded in porous glass or in a polymer film, are unsuitable for many electronic and optical applications. The porous glass compositions are fragile and cannot be machined or polished by the techniques used for standard optical glass. In general, the polymer/semiconductor compositions lack the thermal stability or high optical quality necessary for most electronic and optical applications. For example, it is difficult to make high-quality optical fibers from the polymer/semiconductor composites.
It is an object of the present invention to provide a chemically and mechanically stable dispersion of small semiconductor particles in an optically transparent and mechanically robust rigid matrix. Such materials are expected to have faster optical nonlinearity than bulk semiconductors. Wavelength tuning could be achieved conveniently by controlling the size and concentration of the semiconductor particles. It is a further object of the present invention to provide materials for generating third order nonlinear optical effects.